Corrosion resistant composition and method



United States Patent Oflice 3,520,736 CORROSION RESISTANT COMPOSITION AND METHOD Isaac Laird Newell, Wethersfield, Conn., and William F. Houlihan, Springfield, Mass., assignors to Heatbath Corporation, Springfield, Mass., a corporation No Drawing. Filed July 21, 1966, Ser. No. 566,755 Int. Cl. C23c 1/10; C23f 7/00 US. Cl. 1486.2 22 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the corrosion protection of aluminum and to nitroprusside-containing compositions for imparting such protection.

This invention relates to conversion coating compositions useful for imparting corrosion protective resistant coatings to metals and more particularly, to aluminum and aluminum alloys containing at least about 80% aluminum.

There are a great many conversion coating compositions on the market that are used to apply corrosion protective coatings on the surfaces of various metals. Such coatings are normally applied to aluminum in order to protect it from the effects of oxidation. They are also applied to metals such as aluminum and steel to provide a foundation through which paint can be securely bonded to the metal.

In modern high-speed technology it is frequently essential that the corrosion resistant coating be imparted very quickly. An application time of under three minutes is often necessary and frequently a coating time of from 10 to 60 seconds is essential. There are few known processes and products capable of meeting these stringent requirements. The one drawback of many known products and processes is that the coating imparted in so short a period of time is substantially colorless and substantially invisible to the unassisted eye, making it extremely diflicult for operators of the process to determine quickly whether an adequate coating has been achieved.

The use of the compositions of the present invention facilities the visual determination of coating effectiveness.

The compositions of this invention are employed in aqueous medium and are substantially compatible with treating equipment and methods already in use. In essence, the compositions of this invention comprise a chromium compound, a nitroprusside compound and a fluorine containing compound. Other auxiliary materials can also be added, as will be indicated below.

The nitroprusside compound is the constituent that appears to be primarily responsible for the superior effects obtained by means of this invention. Compositions containing chromium compounds or compositions containing chromium compounds plus fluorine containing compounds have been known for at least about 35 years. Ferricyanide-fluoride compounds have been tried, with and without chromium compounds, but this combination leads to varying colorations and coating thicknesses with relatively small changes, sometimes inadvertent, in ingredient concentrations. The use of a nitroprusside constituent gives greater stability with changes in concentration.

The nitroprusside compounds of this invention should 3,520,736 Patented July 14, 1970 be water soluble. Generally, solubility of the nitroprusside compounds will be at least about one gram of nitroprusside compound per liter of water. The more soluble the nitroprusside compound, the more effective the results. The nitroprusside compounds can be regarded as nitrosoferricyanide compounds and some others alternatively refer to them as nitroprussiates or nitroferricyanides. Of the available nitroprussides, sodium and potassium nitroprussides are preferred. The sodium nitroprusside of commerce, for example, is in the dihydrate form and is usually regarded as having the formula:

The water content of this commercial sodium nitroprusside is approximately 12% by weight. Similarly, commercial potassium nitroprusside is usually obtained in the dihydrate form containing about 11% water.

The chromium compound should be water soluble in the amount employed in the composition. Preferably, the water solubility of the chromium compound should be at least about 500 grams per liter. The chromium in the chromium compound should be in the hexavalent state. Chromium trioxide (CrO is a useful and preferred chromium compound. Chromium trioxide is also referred to in the art as either chromic acid or chromic anhydride, there being some question as to whether in aqueous solution the initial chromium trioxide reacts with water to form the compound that might be referred to as hydrogen chromate or chromic acid, or whether chromium trioxide exists in solution as such. As used in this application and in the claims appended hereto, the term chromium trioxide is intended to refer both to solid chromium trioxide and to the compound formed when chromium trioxide is dissolved in water. Other water soluble sources of hexavalent chromium can be employed instead. For example, alkali 'metal chromates such as sodium chromate and potassium chromate can be employed. Other chromates meeting the solubility requirements can also be employed. Where desired, dichromates such as alkali metal dichromates can be used as the chromium compound or as an auxiliary chromium compound in admixture with chromium trioxide or a water soluble chromate.

The third component of the compositions of this invention is an inorganic fluorine containing compound. The precise identity of the inorganic fluorine containing compound is not critical; although it must be at least sparingly soluble in water and at least partially ionizable. Compounds used in this invention to supply a source of fluorine should be soluble at least to the extent of about 0.1 gram per liter of water. Representative fluorine containing compounds, capable of use in this invention, include hydrogen fluoride, salts of hydrogen fluoride such as the alkali metal salts, e.g. sodium fluoride and potassium fluoride, the fluosilicates (otherwise known as silicofiuorides), such as sodium fluosilicate and potassium fluosilicate. Fluosilicic acid can also be employed if desired; although it is difficult to handle. Other fluorine compounds useful in the invention include bifluorides, for example, sodium bifluoride, NaHF fluoboric acid and the various salts of fluoboric acid, e.g. sodium fluoborate and potassium fluoborate, fluophosphoric acid and salts thereof, fluotitanates, fluozirconates, and other complex fluorides.

In the aqueous coating solution, the chromium compound should be employed in an amount equivalent in chromium content to from about 1.5 to 20 grams, preferably about 2.5 to 10 grams of chromium trioxide per liter of aqueous solution.

The quantity of nitroprusside compound employed should be such that the nitroprusside content, calculated on the basis of the nitroprusside group, of the nitroprusside compound should be present in aqueous solution in an amount from about 0.07 to 12 grams, preferably about 0.1 to 10 grams, per liter of aqueous solution.

The inorganic fluorine containing compound should be present in sufficient quantity such that the fluorine component, calculated on the basis of elemental fluorine, is present in an amount of from about 0.2 to 5 grams per liter of aqueous solution, preferably from about 1 /2 to 3 /2 grams per liter. It will be readily understood that if more than one fluorine compound is employed, for example, the calculation can be made in the same fashion with the fluorine contribution of each of the fluorine compounds being added to obtain the total fluorine content of the solution. By fluorine, in this context, reference is made to the element fluorine in whatever form it may be present and not to fluorine as such in its pure elemental state.

Other additives can be employed in the practice of this invention to enhance the effectiveness of the inventive compositions. For example, improved results appear to be obtained when there is an inorganic nitrate present in the solution. The inorganic nitrate can be nitric acid itself or a water soluble nitrate whose cationic portion does not have an adverse effect on the other constituents of the compositions and processes. Representative inorganic nitrates include sodium nitrate, potassium nitrate and ferric nitrate. The quantity of nitrate component can vary from to about 10 grams per liter calculated on the weight of the nitrate radical. In some instances, the use of nitrate such as ferric nitrate is desirable for added coloration. Other nitrates whose cations form colored products with ferricyanides can also be employed.

The pH of the aqueous solution is from about 0.7 to 2.5, preferably in the range from 1 to 2. Adjustment of pH can be made where necessary with suitable acids such as nitric acid. The use of reducing acids, such as sulfurous acid is best avoided since it appears to have a deleterious effect.

Where alkaline adjustment is required, a suitable alkali metal hydroxide or carbonate such as sodium hydroxide or potassium hydroxide can be employed.

A convenient method of marketing the compositions of this invention is to package the chromium compound, the fluorine-containing compound and the nitroprusside compound all together in solid form in substantially anhydrous conditions with the ultimate user adding the water. Alternatively, the components can be marketed as an aqueous concentrate of chromium trioxide and the nitroprusside, with the user adding the more sparingly soluble fluorine containing compound and additional water to make up the requisite concentration.

In use, the compositions of this invention are particularly adapted for application to an aluminum or aluminum alloy (at least 80% aluminum) surface. Good results are also achieved upon application to other metals with which the compositions are compatible using standard handling and process techniques. Representative other metals include zinc, magnesium and cadmium. All that is necessary is to insure adequate contact between the aqueous composition and the metal surface being protected. A contact time from 5 seconds upward can be employed, dependent upon the thickness of coating desired. Contact times of seconds yield coatings having commercially desirable properties in all respects. Longer contact times can be employed to build up thicker coatings where subsequent finishing operations require, for one reason or another, a thicker coating. The compositions of the invention can be applied by spraying, brushing, immersing, dipping, roller coating, painting or any other suitable means. The temperature of application can preferably be within the range from about 60 F. to 150 F.

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The following examples in the opinion of the inventors represent the best mode of carrying out the invention.

EXAMPLE 1 1 liter of an aqueous solution was made up containing the following quantities of materials expressed in grams per liter.

Component: Grams per liter Chromium trioxide 4.73 Sodium fluosilicate 0.93

Sodium nitroprusside dihydrate 0.93 Sodium nitrate 1.86 Ferric nitrate 0.93 Water to make 1 liter.

The pH of this solution was about 1.5. A 2S aluminum specimen measuring 2 inches by 3 inches by inch was fully immersed in this solution for a period of 10 seconds with the temperature maintained at F. At the end of this period of time the aluminum specimen was removed from the solution and observed to contain a light golden yellow iridescent coating which was visually homogeneous. Upon further testing good corrosion resistant properties were observed to have been imparted by this composition.

EXAMPLE 2 Similar results were obtained with the following composition used at a temperature of F.

Component: Grams per liter Chromium trioxide 9.46 Sodium fluosilicate 1.86 Sodium nitroprusside dihydrate 5.60 Sodium fluoride 1.86 Water to make 1 liter.

EXAMPLE 3 Example 1 was repeated with the attainment of similarly satisfactory results using the following composition at a temperature of 100 F.

Component: Grams per liter Chromium trioxide 4.73

Sodium fluosilicate 0.43 Sodium bifluoride 1.40 Sodium nitroprusside dihydrate 3.20 Sodium nitrate 1.63 Ferric nitrate 1.06 Water to make 1 liter.

EXAMPLE 4 The following composition produced satisfactory results when applied for a period of 30 seconds to a Type 3003 aluminum and Type 24ST aluminum alloy samples at a temperature of 70 F. in aqueous solution.

Component: Grams per liter Chromium trioxide 0.43 Sodium fluosilicate 0.43 Sodium fluoride 1.40 Potassium nitroprusside dihydrate 3.20 Sodium nitrate 1.63 Ferric nitrate 2.13

EXAMPLES 5 THROUGH 13 The following additional compositions were found to yield satisfactory corrosion protection to aluminum samples.

Grams per liter Example 6 7 8 9 10 11 12 13 Chromium trioxide 4. 73 4. 73 4. 73 4. 73 4. 73 4. 73 4. 73 4. 73 4. 73 Sodium nitrate 1. 86 1. 86 1. 86 1. 63 Sodium fiuosilicate. 1. 86 1. 86 0. 93 U. 43 1. 86 1. 86 O. 93 0. 93 l. 86 Sodium nitropi'usside 0. 93 0. 93 0. 93 1. 06 0. 93 0. 93 2. 80 2. 80 l. 86 Ferric nitrate 0. 93 0. 93 0. 93 Sodium bifluoride 0. 93 1. 40 0.93 0, 93 Water to make 1 liter.

Satisfactory corrosion protection was also afforded by 10 5. A composition as in claim 4 also containing an inthe following compositions when applied to Type 3003 aluminum alloy specimens by immersion:

Example Component 14 15 16 Sodium diehromate (g./l.) 8. 6 4.3 Chromium trioxide (g./1.) 3. 3 Sodium silicofluoride. 2. 7 1. 3 1. 3 Sodium nitroprusside..- 1. 3 0. 7 0. 110 Nitric acid (70%)-... 2. 8 1. 4 odium nitrate Water (to make one liter) q.s. q.s q.s. Application conditions:

Temperature F.) 85 7O 70 Time, seconds l 30 180 180 EXAMPLES 17-19 The following solution was prepared:

Grams Chromium trioxide 7 Sodium silicofluoride 0.3 Sodium nitroprusside 0.7 Sodium bisulfate 0.7

Water, to make one liter.

Several different metals were immersed in this solution and observed.

In each case, a generally uniform coating was observed showing a generally golden color. Increasing the quantity of sodium silicofluoride up to 4 grams per liter showed a very gradual decrease in intensity of the gold pigmentation of the coating.

Having thus described the invention, that which is desired to be protected by Letters Patent is as follows:

1. A composition capable of producing a visible protective coating on aluminum comprising an aqueous solution of a chromium compound in which the chromium is in hexavalent form and is present in a quantity expressed as chromium of from about 1.5 to 20 grams per liter, an alkali metal nitroprusside compound which is present in a sufiicient quantity that it furnishes from about 0.07 to 12 grams of nitroprusside to the solution per liter of aqueous solution, and an inorganic fluorine containing compound having a water solubility of at least about 0.1 gram per liter in suflicient quantity to furnish from about 0.2 to 5.25 grams of fluorine per liter of solution.

2. A composition as in claim 1 also containing an inorganic nitrate.

3. A composition as in claim 1 in which the fluorine compound comprises a fiuosilicate.

4. A composition as in claim 1 wherein the chromium compound is present in an amount calculated as chromium trioxide of from about 2 /2 to 10 grams per liter of aqueous solution, the nitroprusside compound is present in sufficient quantity to furnish from about 0.1 to 10 grams of nitroprusside per liter of aqueous solution and the fluorine containing compound is present in sufiicient quantity to furnish from about 1 /2 to 3 /2 grams of fluorine per liter of solution.

organic nitrate.

6. A composition as in claim 5 wherein the fluorine containing compound comprises a fiuosilicate.

7. A composition as in claim 4 wherein the fluorine containing compound comprises a bifluoride compound.

8. A composition as in claim 4 wherein the chromium compound is chromium trioxide, the nitroprusside compound is sodium nitroprusside or potassium nitroprusside and the fluorine containing compound is a fiuosilicate or a mixture of a fiuosilicate and a fluoride.

9. A composition as in claim 8 wherein the fiuosilicate is sodium fiuosilicate or potassium fiuosilicate.

10. A composition as in claim 9 also containing at least one compound from the group consisting of sodium nitrate, potassium nitrate and ferric nitrate.

11. A composition as in claim 1 wherein the fluorine containing compound comprises a bifluoride compound.

12. A composition as in claim 1 wherein the nitroprusside compound is sodium nitroprusside or potassium nitroprusside and the fluorine containing compound is a fiuosilicate or a fiuosilicate and a bifluoride.

13. A composition as in claim 12 wherein the fluosilicate is sodium fiuosilicate or potassium fiuosilicate.

14. A composition as in claim 12 also containing at least one compound from the group consisting of sodium nitrate, potassium nitrate and ferric nitrate.

15. Composition as in claim 1 wherein the chromium is present in an amount calculated as chromium trioxide of from about 2.5 to 10 grams per liter of aqueous solution.

16. A composition as in claim 15 wherein the inorganic fluorine containing compound is a bifluoride.

17. A composition as in claim 1 in which the fluorine compound comprises an alkali metal fluoride.

18. A composition as in claim 1 in which the fluorine compound comprises fluoboric acid.

19. A composition as in claim 1 in which the fluorine compound is fluophosphoric acid.

20. A method of imparting a visible protective film to a metal surface comprising contacting the metal surface with the composition of claim 15 for a time and at a temperature suflicient to cause the formation of a visible protective film.

21. A method as in claim 20 wherein the metal surface comprises aluminum.

22. A method of imparting a visible protective film to a surface containing at least aluminum comprising contacting the metal surface with the composition of claim 8 for a time and at a temperature suflicient to cause the formation of a visible protective film.

References Cited UNITED STATES PATENTS 2,796,371 6/1957 Ostrander et al 1486.2 3,066,055 11/1962 Pimbley l48-6.27 3,348,979 10/1967 Murphy et a1. 1486.2

DONALD J. ARNOLD, Primary Examiner L. B. HAYES, Assistant Examiner US. Cl. X.R. 10614; 117-127; 148-627 

